You get "Mismatched entity and/or field definitions" warnings on a site's Status page
You notice that entity-updates are no longer done by database updates.
Follow fully the advice on the `devel_entity_updates` project page .
In particular it says, at the time of this writing,
Do not use this to fix the
Mismatched entity and/or field definitionserror: again, this is not meant to fix production sites.
If you encounter that error you should identify which module defines the problematic entity type or field type and open a bug report or support request in its issue queue.
The Drupal community changed the workflow for applying entity-updates. See Support for entity updates removed from Drupal core
Now, the `devel_entity_updates` module is needed to apply entity updates AND even with this module on a non-production site, updates cannot be applied to existing content. Hence the advice on the project page. For the future customers need to fix their workflow so that the error does not happen (ideally) but for now more manual database schema changes will be needed to correct existing Prod that has the error, hence finding and contributing to the Issue Queues of the contrib modules that define the entities is important.